1. Field of the Invention
Embodiments of the present invention relate to methods, systems and devices for detecting rotating stall in turbomachinery. A system is provided for detecting the onset of compressor stall, which comprises a line probe for transmitting light toward and receiving reflected light from the surface of a blade moving through the optical path of the line probe and a detector for measuring the intensity of the reflected light to determine a change in blade angle indicative of blade stall. Embodiments of the invention are capable of identifying in real time the onset of compressor stall in both axial and centrifugal flow compressors.
2. Description of Related Art
Rotating stall within a compressor is known to be a factor which can lead to the occurrence of compressor surge. In a normally operating compressor, the air flows predominantly along the axis of the compressor. In the event of rotating stall, a portion of air begins a behavior in which it substantially propagates around the compressor stage with the blades. In this case a small proportion of compressor blades experience flow separation, creating a region of stalled air which circulates around the engine at roughly half the speed of the rotor's revolution. Some key impacts of rotating stall include reduction in engine output power, and excess mechanical loading on the compressor blades. Once compressor stall is initiated, it can spread sequentially to other blades, eventually resulting in the stalling of the majority of the rotor, and leading to surge.
Rotating stall can progress to initiate axi-symmetric stall. In an axi-symmetric stall, the entire compressor stage loses the ability to operate, resulting in a local reversal of the air flow. This in turn can cause the stalling of the preceding stage, and allow the phenomenon to propagate forwards through the engine, resulting in compressor surge. In this case the engine completely loses compression, resulting in a reversal of flow through the engine. This in turn, can cause engine wear, extreme vibration, and even total destruction of the engine. Often the conditions which create the surge remain in place for a period of several seconds, resulting in a repeated surging of the engine.
Compressor surge is a major concern of both the US military and civilian aircraft industries. The phenomenon has been identified as the cause of several incidents that have resulted in the death and injury to passengers, pilots, and staff. Operating the turbine engine so as to avoid compressor stall also leads to a loss in efficiency of the turbine engine, particularly at higher speeds.
There are numerous causes of compressor stall and surge, ranging from attempting to increase the engine speed too rapidly, to ingestion of a foreign object, erosion of engine parts, or extreme maneuvering. Surge can also be caused by an engine ingesting hot gases, during thrust reversal for example. Currently, in order to prevent compressor stall and surge, engines must be operated at sub optimal efficiency, particularly at higher speeds.
By detecting rotating stall, remediation actions may be undertaken to suppress the rotating stall prior to the occurrence of surge. Methods are known for suppressing rotating stall using active controls. One method for performing this suppression is through the injection of high pressure air at specific locations and times. This requires early knowledge of the rotating stall before it spreads to create the surge. Previous work has been performed on trying to detect surge precursors in compressors, such as using high speed pressure measurements as described in the dissertation of Dr. William Cousins, Virginia Tech, 1997. This research indicates that the amount of time required for a rotating stall to evolve into a compressor surge is dependent on the current speed of the compressor, which generally takes at least a couple of revolutions, and at lower speeds, the time increases further.
A known approach for detecting the onset of compressor stall is disclosed in U.S. Pat. No. 6,474,935 entitled “Optical Stall Precursor Sensor Apparatus and Method for Application on Axial Flow Compressors.” This approach measures the time of arrival of the tip of an airfoil and compares this data with a predetermined fixed value. If the measurement is greater than the reference, the system concludes a stall condition is present. Time of arrival of the blade tip, however, can be dependent on many factors and is not necessarily an accurate indicator of compressor stall. For example, the time of arrival of a blade tip can be altered when a blade is damaged by a domestic or foreign object. As a result, there may be a defect in the blade tip that alters its time of arrival but that does not contribute to compressor surge. If the change in time of arrival is above the fixed threshold, then a false indicator of surge will result. A more accurate time of arrival based measurement would take into account a plurality of data points on the blade surface rather than just the blade tip. Further, comparing the change in time of arrival of a blade tip to only a fixed pre-determined reference may not take into account changing conditions of the rotor during operation. Time of arrival of the blade tip may change during operation as a natural result of the operating conditions. If the pre-determined reference value is not updated to reflect the new operating conditions, then again a false indication of compressor stall may result.
Other approaches like those described in U.S. Pat. Nos. 7,783,395 and 7,905,702 use sensors to identify a stall condition by detecting a change in pressure flow resulting from stalled air circulating around the rotor instead of through the rotor. Such approaches detect stall when it has already occurred because the decrease in pressure flow only occurs as a result of stall. These systems therefore are unable to provide a control signal to initiate procedures to prevent the stall from occurring.
What is lacking is an early warning sensor capable of accurately detecting and preventing a stall condition prior to compressor surge. A reliable system is needed that monitors multiple points on a blade and compares the position of those points with reference values obtained from other blades, where the reference values are updated to account for changing conditions of the compressor during operation.